An inkjet printing system typically includes one or more printheads and their corresponding ink supplies. Each printhead includes an ink inlet that is connected to its ink supply and an array of drop ejectors, each ejector consisting of an ink pressurization chamber, an ejecting actuator and a nozzle through which droplets of ink are ejected. The ejecting actuator may be one of various types, including a heater that vaporizes some of the ink in the pressurization chamber in order to propel a droplet out of the orifice, or a piezoelectric device which changes the wall geometry of the chamber in order to generate a pressure wave that ejects a droplet. The droplets are typically directed toward paper or other recording medium in order to produce an image according to image data that is converted into electronic firing pulses for the drop ejectors as the recording medium is moved relative to the printhead.
A common type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the recording medium and the printhead is mounted on a carriage. In a carriage printer, the recording medium is advanced a given distance along a media advance direction and then stopped. While the recording medium is stopped, the printhead carriage is moved in a direction that is substantially perpendicular to the media advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the recording medium, the recording medium is advanced; the carriage direction of motion is reversed, and the image is formed swath by swath.
The ink supply on a carriage printer can be mounted on the carriage or off the carriage. For the case of ink supplies being mounted on the carriage, the ink tank can be permanently integrated with the printhead as a print cartridge, so that the printhead needs to be replaced when the ink is depleted, or the ink tank can be detachably mounted to the printhead so that only the ink tank itself needs to be replaced when the ink tank is depleted. Carriage mounted ink supplies typically contain only enough ink for up to about several hundred prints. This is because the total mass of the carriage needs be limited so that accelerations of the carriage at each end of the travel do not result in large forces that can shake the printer back and forth.
Pickup rollers are used to advance the media from its holding tray along a transport path towards a print zone beneath the carriage printer where the ink is projected onto the media. In the print zone, ink droplets are ejected onto the media according to corresponding printing data.
It is noted that consumers use a plurality of different types of media for printing in inkjet printers. Commonly assigned and pending U.S. patent application Ser. No. 12/959,461 uses a sensor having a light source and detector for detecting the type of media being used for printing. As with any light source, light intensity may vary slightly over time causing the resulting signal used for detecting the media type to correspondingly vary.
Although the currently used apparatuses and methods for detecting the media type are sufficient, there exists a need to detect such light variations using transmissive optics and to calibrate the photo-detector signal accordingly for permitting accurate detection of media type. Consequently, the present invention provides a method for detecting the light variation and providing a calibration signal.